Nostradamus might have received his prophecies verbally. He heard a voice uttering prophecies.

How did Nostradamus manage to receive prophecies?

The Bible spoke also of a spirit that 'always spoke with a low voice':

'And they shall say unto them that have familiar spirits that peep and mutter ...
and your speech shall be low out of the dust, and your voice shall be of one that has
a familiar spirit.'

(Isaiah 8.19, 29.4. See also Deuteronomy 18.9-14.)

Nostradamus did not mention any physical experience of the future. He did not go there,
he just heard a voice uttering prophecies.

Nostradamus refers to a spirit,
something you cannot see (or might only be visible to one person).
Although you do not see a spirit, you might be able to communicate with it, an activity
that could resemble telecommunication, radio transmission or broadcasting.
In a way you could say that if Nostradamus did receive his predictions this way,
then he did not go out to get them, pointing to the possibility that he was a more or less
passive listener. Consequently, it then also means that there should be a broadcaster
somewhere else, something or someone that has been the source of information.

Did Nostradamus maybe listened to some kind of broadcasting and wrote it down in his own words?

In the literature on Nostradamus, he himself is the central figure.
Little or no attention is given to the source of his knowledge.
That is understandable, since we do not really know what this source might have been.
Still it seems that he depended on it for his predictions.

I suggest that the first thing we have to do is to stop thinking of Nostradamus
as someone special who had a gift. He might have been as ordinary as the rest of us.
From his biography it is known that he is not that different from contemporary French or other
human beings in general. And as far as we know normal people do not have the ability to make true
predictions.

If we still believe that Nostradamus really based his books on true prophecies.
Than we should focus on the occult source
of his knowledge.

When mentioning the predictions of Nostradamus
We should think of it as two parts. The first part is about the writer Nostradamus,
his task was limited, receiving visions and writing them down.
What we know about him does not explain where the visions came from.
The second part is about the occult source, the sender and creator of the predictions.

If we analyze with this attitude, it is obviously that the occult source must have played
a far more prominent role than we might realize.